London has been battered by 50mph winds that have felled trees and caused travel chaos. Powerful gusts swept across the capital as the Met Office issued a yellow "be aware" weather alert for most of the country.

The first question Richard Mason asks me before our interview begins is whether he should change for the photographs out of his tight-fitting T-shirt and jeans into something more formal, as befits a bestselling author doing the rounds to publicise his latest novel.

But as the book, History of a Pleasure Seeker, is a ripping literary romp about the adventures of a dashing, athletic and sexually ambiguous young man, I tell him he looks just right as he is.

Mason, who is 33, dark-haired and rather handsome, has just flown in from New York, where he has recently moved. He's one of those people who never settles anywhere for long. Born in South Africa, educated at Eton and Oxford, and with several homes dotted about the globe, he and his partner Benjamin Morse - an American theologian academic six years his senior - are doing a short stint in New York to produce a deluxe digital iPad edition of the new novel.

He's not a household name but he ought to be, since his first novel, The Drowning People, published when he was just 21, has sold over three million copies worldwide and won the Italian Grinzane Cavour Prize for Best First Novel. Since then, he has written two more commercially successful novels, and The Drowning People is currently in production being made into a movie.

"I'm probably not meant to tell you this but Blake Lively from Gossip Girl is my number one choice to play the lead," he says conspiratorially. Indeed, he admits that he and Morse prefer to spend evenings at home watching Gossip Girl rather than participate in New York's famously raunchy gay scene.

"You can throw yourself into a life of debauched hedonism or you can live a sober life of self-improvement, meditation, personal trainers and 12-step programmes. I'm trying to stick to the second, with just a little bit of the first for fun," he says.

Last year he and Morse spent several months living in a tent in rural South Africa, setting up the Lulutho project: teaching farming communities about climate change and sustainability. And before that they lived in Glasgow, where in 2007 they tied the knot in a CofE ceremony. But Mason, who grew up in Johannesburg during apartheid, came to the UK aged 10 with his political dissident parents, leaving three much elder siblings behind.

A year later he was sent to Eton where he excelled academically but struggled with his sexuality.

"I've always felt my own sexuality quite intensely and known it's important for me to square what it is with the outside world. I remember thinking I'd come from a colonial, homophobic background and Eton was just the same. I was an outsider at this posh English school and spent my adolescence trying to figure out how to fit into this crazy world. I never got up to anything there myself but I later discovered that the most homophobic boys were always the ones having sex with the other boys."

It was only when Mason went to Oxford and began "an intense and explosive" relationship with Morse that he was forced to come to terms with his sexuality. The pair were living together, which was unusual in those days, and he had heard that the student newspaper Cherwell was planning to run a big feature on "the only gay couple in Oxford".

"I don't think people saw how difficult it was for me. I found it gruelling and torturous. But it was my own internalised hostility towards homosexuality that had definitely come from being part of an English boarding school culture."

Thankfully, he says, the culture in this country has radically changed, for which he credits the Blair government. "One thing it did really well was to stand up for civil rights. I think the legalisation of gay partnerships has altered Britain profoundly in really beneficial ways.

Every time I'm in London I see more same-sex couples holding hands and not just in Old Compton Street," he says. "Which is why people like radio presenter Chris Moyles really annoy me. It's not OK to use the word gay to mean you're a wanker."

Mason goes on to say he doesn't really believe in the concepts of straight or gay. "I've always felt tugged in both directions. At my wedding there were six of my ex-girlfriends. The best man made an amusing speech about them barking up the wrong tree but they hadn't really barked up the wrong tree. I had girlfriends. I wasn't pretending either " He pauses, lowering his voice. "I can't believe I'm saying this but I really enjoyed having sex with them "

The main character in his new novel - who will feature in what Mason calls his forthcoming "galaxy" of several books - also enjoys a richly varied sex life. An opportunistic but essentially goodhearted rake, he lands a job as a tutor in a rich family in Amsterdam at the turn of the 20th century, and his efforts to satisfy the needs of the mistress of the house, while fending off the randy footman, are all unflinchingly described by Mason, who was inspired by reading Victorian pornography. "If you read the diaries and private literature of the past it completely confirms that this division of gay and straight - with maybe a bisexual compartment - really only happened at the time of the trial of Oscar Wilde." He decided against using euphemisms too. "No hairy diadems. It's an arm. It's a leg. It's a cock."

He was also profoundly touched by Irène Nemirovsky's Suite Francaise, "one of the jewels of 20th-century writing" and written by someone "who knew she didn't have much time, paper or ink". He wrote the entire novel in longhand, straight into a blue leatherbound notebook, in 18 months. He shows it to me proudly. Words are crossed through and sentences neatly deleted. "Microsoft does not encourage concision, but longhand does," he says.

Which was just as well, since Mason's tendency to relentless perfectionism is symptomatic, along with bouts of depression and manic elation, of the bipolar disorder that runs in his family and caused his older sister Kay to commit suicide 23 years ago. Mason himself suffered a major breakdown eight years ago, brought on by stress and overwork, and he might have seriously contemplated killing himself had he not had the support of his parents and Morse.

"The biggest thing is not about not going down," he says, "it's about not going up too. Once I recognised the link between soaring exhilaration and what it's like at the bottom end of that curve, the elation didn't seem fun at all."

Today he manages the condition by taking the mood-stabilising, anti-epileptic drug Lamotrigine every day and ensuring that he eats, sleeps and exercises regularly. He eats up to five bananas a day, keeps his alcohol consumption down, and if he feels stressed or complacent, he stops whatever he's doing and takes a nap. It's a strategy he learned from the NHS psychiatrist who treated him after his breakdown and with whom he stays in regular touch.

He has nothing but praise for the "brilliant NHS". He also disagrees with Stephen Fry's belief that taking medication causes those creative surges to be lost. On the contrary, he believes calmness leads to greater creativity.

As a result, Mason has not felt suicidal for eight years, and the panic attacks which used to paralyse him have all but gone, although, he confides he had a small one just before he left New York. But rather than get in a flap about the book tour, he went to bed and slept.

He only wishes such understanding of what he calls a bipolar "disposition" had existed when Kay was still alive. However, the Kay Mason Foundation, which Mason set up in her memory with proceeds from his books, is flourishing. It awards scholarships to disadvantaged children in South Africa whose families are unable to afford school fees, and with the patronage of Desmond Tutu and contributions from the Prince of Wales, among others, it has grown from awarding four scholarships a year to 100.

In spite of not having had a holiday for four years, he feels calmer than ever. "Let's call it keeping at 6.5 on the scale, which is a much more creative place than oscillating between nine and one."